Tidal velocity asymmetries and bedload transport in shallow embayments (original) (raw)

A study of non-linear tidal propagation in shallow inlet/estuarine systems Part II: Theory

Estuarine Coastal and Shelf Science, 1985

The offshore tide becomes strongly distorted as it propagates into shallow estuarine systems. Observations of sea surface elevation and horizontal currents over periods ranging from three days to one year, at nine stations within Nauset inlet/estuary, document the non-linear interaction of the offshore equilibrium tidal constituents. Despite strong frictional attenuation within the estuary, the overtides and compound tides of M,, S, and N,, in particular, reach significant amplitude, resulting in strong tidal distortion. High frequency forced constituents in sea surface are phase-locked, consistently leading the forcing tides by 60-70", resulting in a persistent distortion where falling tide is longer than rising tide. Forced constituents in currents are more nearly in phase with equilibrium constituents, producing flood currents which are shorter but more intense than ebb currents. A compound fortnightly tide, MS,, modulates the mean water level such that lowest tides occur during neap phase instead of spring phase. This fortnightly tide can be contaminated by storm surge, changing the phase characteristics of this constituent. Implications of the overtides, compound tides, and lower frequency tides on near-bed, suspended and dissolved material transport are profound.

Bedload transport in an inlet channel during a tidal cycle

Proceedings of the 5th IAHR Symposium on River, Coastal and Estuarine Morphodynamics, Enschede, NL, 17-21 September 2007, 2007

Based on high-resolution swath bathymetry measurements at centimetre-scale precision conducted during a tidal cycle in a dune field in the Grådyb tidal inlet channel in the Danish Wadden Sea, a simple tool to calculate bedload transport is presented. Bedload transport was related to simultaneous flow measurements using an acoustic Doppler current profiler. Spatially, bedload transport was higher on the dune crests than in the dune troughs of the large ebb-directed compound dunes, due to higher flow velocities at the crests than in the troughs. Temporally, bedload transport was higher in the troughs of the large ebb-directed compound dunes during the flood tide than during the ebb tide, due to higher near-bed flow velocities in the troughs of the compound dunes during the flood tide as compared to the ebb tide, resulting from flow exposure, i.e. the trough-sheltering effect of the compound dunes being active during the ebb tide, but negligible during the flood tide. Bedload transport was also predicted using five classical and widely used formulae. These predictors were all unable to depict the temporal variation in bedload transport during the tidal cycle. It is suggested that temporal variations in grain-size composition of the mobilised sediment should be taken into account by sediment transport formulae.

A study of non-linear tidal propagation in shallow inlet/estuarine systems Part I: Observations

Estuarine Coastal and Shelf Science, 1985

Geometry of tidal inlet systems: A key factor for the net sediment transport in tidal inlets

Journal of Geophysical Research: Oceans, 2014

The net transport of sediment between the back-barrier basin and the sea is an important process for determining the stability of tidal inlet systems. Earlier studies showed that in a short basin, tidal flats favor peak ebb-currents stronger than peak flood currents, implying export of coarse sediment, while shallow basins favor stronger flood currents. The new elements considered in this study are 1) arbitrary basin lengths, 2) a narrow inlet that connects the basin to the sea, 3) an asymmetric tidal forcing, and 4) radiation damping. The objective is to gain fundamental insight in how the geometry of a tidal inlet system affects the net sand transport in a tidal inlet. For this purpose, a width-and depth averaged analytical model was constructed. It is found that the length of a back-barrier basin controls the effect that nonlinear hydrodynamic processes have on the tidal asymmetry, and consequently controls whether the currents in the inlet are flood-or ebb-dominant. Furthermore, the cross-sectional area of the inlet controls the ratio between the net sediment transport that results from tidal asymmetry and that caused by the interaction of the principal tide with the residual current. Finally, it is shown that the effect of an asymmetric tidal forcing on the net sand transport depends on the length of the back-barrier basin with respect to the tidal wavelength in that basin.

Numerical simulation of tide-induced transport of heterogeneous sediments in the English Channel

Continental Shelf Research, 2010

The three-dimensional numerical model COHERENS (COupled Hydrodynamical-Ecolo -gical model for REgioNal and Shelf seas) has been adapted to compute the rates of transport as bedload and suspended load of heterogeneous bottom sediments induced by the dominant M 2 tide in the English Channel. A pre-processing of an extensive surficial sediments dataset has been performed to determine the seabed composition (grain-size distribution or presence of rocks) at the computational grid nodes. Maximum bedload and suspended load transport rates over the tidal cycle, as well as the contributions of the 10 different sedimentary classes to the mean transports are computed. Highest sediment transport rates occur in fine sediments areas located in the surroundings of high shear stresses areas. Medium sand (d 4 = 350 µm) is found to be predominant in bedload, while suspension load implies mainly silts (d 1 = 25 µm) in the inner shoreface and both fine and medium sands (d 3 = 150 µm, d 4 = 350 µm) in the outer shoreface. The offshore residual bedload transport pathways are orientated westerly in the western part of the Channel and easterly in the eastern part defining a "parting" zone which runs from the Isle of Wight to the Cotentin Peninsula. An offshore "bedload convergence" occurs in the southwest of the Dover Strait; a narrow transport pathway bypassing it along the French coastline. These features reproduce those predicted by and provide higher resolution features like inshore headland-induced gyres, particularly along the English coastline. The new predicted general pattern of residual suspended load transport is very similar to the bedload pattern. Differences arise in the central "divergence" zone which exhibits a "Y" shape with two branches ending on both sides of the Isle of Wight, in the Baie de Seine characterized by a central "convergence" and along the English coastline studded with many headland-induced recirculations.

Hydrodynamics and sediment transport in a salt marsh tidal channel

Proceedings of the 16th Coastal Engineering …, 1978

Processes and sediment transport were investigated in a salt marsh drainage system at Kiawah Island, South Carolina. A general survey of the tidal current was done in the major tidal channel (Bass Creek) for a 10 tidal cycle period in August, 1977. Detailed determinations of current velocity, discharge, and suspended load were conducted during 15 tidal cycles in March, 1977 and again during 8 tidal cycles in July-August, 1977. For each of these periods, mass budget for the total suspended load were computed. The tidal currents have a pronounced time velocity asymmetry with the maximum current velocity occurring nearer high slack water and the peak ebb velocity being 20-30% stronger than the flood. Suspended load transport is significantly affected by the time velocity asymmetry. Peak current occurring nearer high slack water causes a net displacement of suspended material in an ebb or seaward direction under normal conditions. This process is enhanced by the stronger ebb currents. Mass budgets reflect the ebb dominance of the system showing a net export of combustible (organic) material during the March sampling period and a net export of both noncombustible (inorganic) and combustible material during the July-August period. Also important to suspended load transport in marsh systems are stressed meteorological conditions. High winds or heavy rains increase suspended load concentration and can cause significant import or export of fine-grained material.

Estimation of annual bedload flux in a macrotidal estuary, Bristol Channel, U. K.

Sedimentological data from the Bristol Channel indicate that “mutually evasive” ebb- and flood-dominant zones of net bedload transport are present. To test this hypothesis, long-term observations of currents from the channel are combined with empirical formulae to estimate annual bedload transport rates under the combined influence of tidal currents and surface wind-waves. Extrapolating these values over the ebb- and flood-dominant zones, sand flux to the upper part of the channel is estimated at about 64 × 105 t yr−1 with supply and removal being of nearly equal magnitude. Such patterns of sand and water movement may be typical of tidally dominated estuaries and embayments.

Numerical study of sediment transport on a tidal flat with a patch of vegetation

Ocean Dynamics, 2014

Your article is protected by copyright and all rights are held exclusively by Springer-Verlag Berlin Heidelberg. This e-offprint is for personal use only and shall not be selfarchived in electronic repositories. If you wish to self-archive your article, please use the accepted manuscript version for posting on your own website. You may further deposit the accepted manuscript version in any repository, provided it is only made publicly available 12 months after official publication or later and provided acknowledgement is given to the original source of publication and a link is inserted to the published article on Springer's website. The link must be accompanied by the following text: "The final publication is available at link.springer.com". Abstract To understand how vegetation canopies affect sediment transport on tidal flats, a numerical study of tidal flow and sediment transport on an idealized tidal flat with a patch of vegetation is conducted. The numerical model is firstly validated by laboratory measurements of flow and sediment deposition in a partially vegetated open channel.

Near-bed sediment transport in a heavily modified coastal plain estuary

International Journal of Sediment Research, 2014

Numerous estuaries of the world have been strongly modified by human activities. These interferences can make great adjustments of not only sediment transport processes, but also the collective behavior of the estuary. This paper provides a typical case of a heavily modified coastal plain estuary of Sheyang on the China coast, where a sluice barrage was built in 1956 to stop the intrusions of storm surges and saline water. Four sets of instrumented tripods were simultaneously deployed along a cross-shore transect to continuously observe near-bed flow currents and sediment transport. The in-situ surveys lasted over a spring and neap tide cycle when a strong wind event occurred in the neap tide. Comparisons of flows and sediment transport between tide-dominated and wind-dominated conditions demonstrated the important role of episodic wind events in flows and sediment transport. The wind-induced currents, bottom stresses, and sediment transport rates were significantly greater when wind was present than corresponding quantities induced by the tides. The long-shore sediment transport induced by winds exceeds the cross-shore component, especially near the river mouth bar. These results indicate the noticeable importance of wave-dominated coastal processes in shaping topographic features. A regime shift of estuarine evolution under highly intense human forcing occurs from fluvial to marine processes. This finding suggests that the management strategy of the estuarine system should focus on the restoration of estuarine processes, rather than the present focus on inhibition of marine dynamics.

Tidal velocity asymmetries and bedload transport in shallow embayments (original) (raw)

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